Aquatics Unlimited: Articles: Filter Carbon
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Filter Carbon

for the week of 3/11/99

What does filter carbon do? Carbon has the ability to "catch" certain chemicals that occur in water (or air, for that matter) by "adsorption". In aquariums, carbon's chief role is to tie up the chemicals that cause discoloration and odors, which are then removed from the system periodically as the carbon is discarded.

Are some carbons better than others? Yes, some filter carbons (the coarse, shiny black stuff) are no more than anthracite coal, and have very limited adsorptive properties. Other carbons are "activated," meaning they were exposed to extreme heat and/or steam to increase their effectiveness. These dull, lighter, carbons may have hundreds of times the capacity of standard coal.

Should everyone use the highest quality carbon? Probably not. Many hobbyists use low to medium grades of carbon, but either use it in large quantities or change it often. Others have high-flow filters that can grind the softer, high quality carbons to dust, which is then blown into the aquarium. On the other hand, some filters hold only small amounts of carbon, so better grades should be used.

Do carbons get full? Yes, eventually the filter carbon has bonded all the chemicals that it can handle. How long that takes depends on the quality and quantity of carbon, the load of fish waste, and several lesser factors. If the aquarium water is taking on an odor or is yellowing, the carbon is full.

Can carbon be re-activated? In a word, no. Heating carbon in a household oven might reclaim a tiny fraction of carbon's power, but probably not enough to pay for the fuel to heat it. It is best to discard used carbon.

How often should carbon be changed? Once carbon is full, it served very little purpose in the aquarium. (It does become a site for good bacteria to colonize and break down ammonia and nitrite, but there's generally no shortage of such sites elsewhere.) Furthermore, some of the compounds adsorbed by the carbon will eventually break down and be released into the aquarium. Carbon should certainly be changed when colors or odors in the water indicate that it has become exhausted.


"Tip of the week" appeared regularly in 1999 and 2000.

Copyright © 1999 Aquatics Unlimited
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